1. Field of the Invention
This invention relates to a magneto-optic recording medium designed to overwrite data directly through optical modulation.
2. Description of Related Art
Referring to FIGS. 1, 2 and 3, there respectively are indicated a perspective view substantially illustrating the structure of a conventional optical recording/reproducing apparatus disclosed, for example, in a draft for a speech at the 34th Applied Physics Related Convention, spring in 1987, 28P-ZL-3, a cross-sectional view showing the optically recording/reproducing state of a conventional magneto-optic recording medium and a graph showing the characteristic of the change of the laser beam power for recording in the area of the conventional magneto-optic recording medium.
A magneto-optic recording medium 1 (hereinafter referred to as a recording medium 1) in FIGS. 1-3 is comprised of a substrate 2 made of glass or plastic, a first magnetic layer 3 made of e.g., Tb.sub.21 Fe.sub.79 and a second magnetic layer 4 made of, e.g., Gd.sub.24 Tb.sub.3 Fe.sub.73. The recording medium 1 is rotated in a direction shown by an arrow a of FIGS. 1 and 2 by a driving mechanism (not shown). The first magnetic layer 3 is similar to a recording layer and a read-out layer in this invention. The second magnetic layer 4 is called an auxiliary layer, works so that an overwriting function, that is, a function to overwrite a new data on an old data at real time is effected, which will be described later. The exchange coupling force exerted between the first and second magnetic layers 3 and 4 is effective to make the direction of magnetization of the first magnetic layer 3 coincident with that of the magnetic layer 4. Supposing that the first and second magnetic layers 3 and 4 have the Curie points T.sub.c1 and T.sub.c2, the coercive force around the room temperature H.sub.c1 and H.sub.c2, and the exchange coupling force at the room temperature H.sub.w1 and H.sub.w2, respectively, the following formulae are satisfied; ##EQU1##
An objective lens 5 is provided above the recording medium 1. It condenses a laser beam and forms a condensing spot 6 on the recording medium 1. A bias magnet 8 confronts to the objective lens 5 to generate a magnetic field of 200-600 Oe, with the recording medium 1 between the objective lens 5 and the bias magnet 8. Further, an initializing magnet 9 is provided at the upper side than the bias magnet 8 in the rotating direction of the recording medium 1, which generates about 5 KOe magnetic field to initialize the second magnetic layer 4. In FIG. 1, the left side to the one-dot chain line represents a new data (DN) area, while the right side to that represents an old data (DO) area. Meanwhile, numeral 7 in FIG. 2 designates an area where a binary-coded data is "1" having the direction of magnetization of the first magnetic layer 3 oriented upwards. Furthermore, an axis of ordinate in the graph of FIG. 3 expresses the laser beam power, and an axis of abscissa represents the area, with R.sub.1 and R.sub.o representing the laser beam power for recording the data "0", respectively.
The operation of the recording medium 1 will be discussed hereinbelow.
In the first place, description is directed to the reproducing operation of data recorded in the recording layer, that is, first magnetic layer 3. As shown in FIG. 2, the first magnetic layer 3 is magnetized upwards or downwards in a thicknesswise direction corresponding to the binary code, namely, "1" and "0". For reproducing the recorded data, the direction of the magnetization of the first magnetic layer 3 at the condensing spot 6 is converted to optical data by the conventionally known optical Kerr effect to detect the data in the recording medium 1. At this time, the power of the laser beam radiated to the recording medium 1 is corresponding to a power A shown in the graph of FIG. 4, which shows a temperature change of magnetic layer in spot in relation to laser beam power. When the recording medium 1 is radiated at the condensing spot 6 by the laser beam of this power, the maximum temperature of each of the first and second magnetic layers 3 and 4 does not reach its Curie point T.sub.c1 or T.sub.c2. Therefore, even when the recording medium is radiated by the laser beam of this power, the direction of magnetization, i.e., recording data is never erased.
Overwriting operation will now be discussed. The initializing magnet 9 of FIG. 1 generates a magnetic field of the intensity H.sub.ini in a direction shown by an arrow b (upwards). The relation of the magnetic field H.sub.ini to the coercive forces and exchange coupling forces of the first and second magnetic layers 3 and 4 is expressed as follows; EQU H.sub.c1 --H.sub.w1 &gt;H.sub.ini &gt;H.sub.c2 +H.sub.w2
Accordingly, when the recording medium 1 is rotated in the direction of arrow a and passes through the position of the initializing magnet 9, the direction of magnetization of the second magnetic layer 4 is made upwards irrespective of the magnetization direction of the first magnetic layer 3. At this time, the magnetization direction of the first magnetic layer 3 is maintained as it is without being influenced by the magnetic field generated by the initializing magnet 9 or by the exchange coupling force exerted by the second magnetic layer 4 at around the room temperature.
In recording the data "1", i.e., when the direction of the magnetization of the first magnetic layer 3 is made upwards, the power of the laser beam corresponds to a power B in FIG. 4. At this time, the temperature in the condensing spot 6 is raised to exceed the Curie point T.sub.c1 of the first magnetic layer 3, but does not reach the Curie point T.sub.c2 of the second magnetic layer 4. Accordingly, although the magnetization of the first magnetic layer 3 is erased, the direction of magnetization of the second magnetic layer 4 is maintained upwards as oriented by the initializing magnet 9. Subsequently, when the recording medium 1 is further rotated with the condensing spot 6 away and the temperature of the first magnetic layer 3 becomes lower than the Curie point T.sub.c1, the direction of magnetization of the second magnetic layer 4 is transferred to the first magnetic layer 3, whereby the first magnetic layer 3 is magnetized upwards, that is, in the direction corresponding to the data "1".
On the other hand, in recording the data "0" to orient the magnetization of the first magnetic layer 3 in the downward direction, the power of the laser beam corresponds to a power C shown in FIG. 4. The temperature in the condensing spot 6 is raised, exceeding not only the Curie point T.sub.c1 of the first magnetic layer 3 but the Curie point T.sub.c2 of the second magnetic layer 4. As a result, both the first and second magnetic layers 3 and 4 are demagnetized in the condensing spot 6. When the recording medium 1 is further rotated, with the condensing spot away and the temperature of the second magnetic layer 4 becomes lower than the Curie point T.sub.c2, the magnetization direction of the second magnetic layer 4 is made downwards by a weak magnetic field applied by the bias magnet 8 in a direction shown by an arrow c in FIG. 8. In the succeeding step where the temperature of the first magnetic layer 3 becomes lower than the Curie point T.sub.c1, the magnetization direction of the second magnetic layer 4 is transferred to the first magnetic layer 3, whereby the direction of magnetization of the first magnetic layer 3 is made downwards, i.e., in the direction corresponding to the data "0".
Thus, by changing the power of the laser beam to a power B or C in accordance with the binary code "0" or "1" to be recorded, a new data can be overwritten at real time on the old data.
Since the conventional magneto-optic recording medium is constituted in the above-described structure, the inverted magnetic field of the second magnetic layer at the room temperature is as large as 3 KOe, whereby an initializing magnet able to generate at least 3 KOe or larger magnetic field is necessary. Accordingly, the conventional optical recording/reproducing apparatus using the magneto-optic recording medium is disadvantageously bulky in size and is not free from adverse influences given by the large initializing magnetic field.